I have now used this bottle on 2 occasions, and was sick with diarrhoea/ vomiting every time. I did use clear and clean looking river water both times, there were no obvious particles in the water to clog up the filter, and nothing to suggest that the water was particularly contaminated (and even if it had been - according to the filter description it should not have been an issue!). I was the only one in the family using the bottle, and the only one affected - so no other likely cause for the symptoms. I really was hoping for an easy solution to the drinking water problem on longer walks, but this is not it for me - in future I will go back to carrying my supply again. May be a little more weight, but at least I won't have to cut my trip short again.